The present invention relates to a tooth contour structure for large sprocket set of bicycle, especially to a kind of unique auxiliary structure arranged on a large sprocket for a bicycle large sprocket set, by the guidance of a speed converter, when the chain is made to be engaged with different sprockets for proceeding the shifting motion, an auxiliary chaining-up is added to raise the shifting efficiency for a tooth contour structure for large sprocket set of bicycle.
Nowadays, all bicycles capable of proceeding multi-section speed changing evenly have been equipped with a large sprocket set rotated by a pedaling crank and arranged with a sprocket wheel set on the rear wheel. Wherein the large sprocket set is usually assembled by at most three sprockets of different sizes, while the sprocket wheel set is then commonly assembled by at least five sprockets of different sizes. The connection and transmission between the large sprocket set and sprocket wheel are carried out by a chain, and matched with the control of front and rear speed converters, the chain is made to be shifted to and engaged with the sprockets of different sizes for the large sprocket set and sprocket wheel set to reach the purpose of speed changing for a bicycle.
Since the difference for the tooth number between the larger sprocket and smaller sprocket of a bicycle large sprocket set is very large, so usually an appropriate device for chaining-up is needed to be arranged on the larger sprocket for making the chain be able to effectively be shifted-up from the smaller sprocket to larger sprocket accurately. Three methods are applied by prior arts for assisting chaining-up: the first kind of method is that by a structure of attached teeth, or a manner of split and projected teeth, or a manner of attached rivets, during the procedure for shifting-up the chain, the attached teeth, rivets, or split teeth may be used for holding the outer chain plate of the chain to make the chain be able to be raised up during the running procedure of the sprockets to reach the object of chaining-up and shifting-up. For example, U.S. Pat. No. 5,192,248 is then one example of them. However, since additional added material of different parts such as rivets etc. are needed for these kinds of structures of attached teeth or attached rivets and additional procedures are also needed for stamping and riveting, so obviously manufacturing procedures would be influenced and lots of manufacturing costs would be increased, and for the latter section of treatment of sprocket product (i.e. tempering or surface anode treatment), technique costs are also caused to increase because of the materials difference for the rivets or attached teeth with the sprockets or the influence of processing stress. Additionally, because the split teeth of the prior art further needs the method of forging for splitting, thinning, and sharpening the teeth, not only a forge with high tonnage is needed for forging and the processing is more difficult, but also the usage life of the die applied for the forging is shortened and its cost is higher, and the split teeth structure also would influence the part""s strength for taking force and safety.
Furthermore, since the chain is alternatively comprised of an external and an internal chain pitch, so in the actual motion of shifting, there will be two different engaging states because of the different phases of engagement between the chain and sprockets, and that causes two different paths for shifting, and the only difference for two kinds of shifting path is the interchange between the external and internal chain pitch. When the auxiliary chaining-up structure of the aforementioned first prior art is facing this problem of two different kinds of chain shifting path, a set of attached teeth, attached rivets, or split teeth is additionally arranged at appropriate position of the different phases on the sprockets, when the chain can not successfully be shifted at the first teeth set because of inappropriate shifting path, the sprocket must continue to rotate for letting the chain be chained-up at the position of next set of attached teeth, attached rivets, or split teeth, but therefore shifting is caused to delay and the efficiency of shifting is lowered.
The second auxiliary chaining-up structure for the prior arts is that several convex points structure is formed by manufacturing procedure of stamping on appropriate positions of the larger sprocket, and the function of the convex points basically are completely same as that of the aforementioned rivets, which is also to raise the chain for auxiliary chaining-up, for example, the U.S. Pat. No. 5,413,534 is one of them. Although this convex points structure of the prior art may be manufactured by the stamping manufacturing procedure of lower cost, but its biggest shortcoming is that this kind of convex points structure is very easily occurred wearing-out phenomenon and caused the failure of shifting, so commonly it is adapted for second class products of low price. Since, when convex point is manufactured by stamping, the profile of the convex point must be arc-shaped, so during the procedure of shifting and chaining-up for the chain, when the chain is raised up by the outer chain plate butting against the convex point, while the chain bears larger load (i.e. the bicycle is under the states of upper-hill or urgent speeding-up), the chain will be very easily glided off the convex point, when the chain is glided off, the convex point is caused to be worn out, and effect for raising up the chain is further worse, therefore the chain is caused to be glided off more easily, and a bad circulation is become, and a further improvement is needed.
The third kind of auxiliary chaining-up structure for the prior art is that a xe2x80x9cparallel moving toothxe2x80x9d structure with which central lines for front and rear tooth faces and tooth top all moving in parallel toward the smaller sprocket is formed by stamping several teeth of the larger sprocket in the direction toward the smaller sprocket to become a structure of auxiliary chaining-up for a chain. Although this kind of parallel moving teeth structure manufactured by stamping has the advantage of lower cost and may be theoretically adapted for two different kinds of shifting path for chain to raise the efficiency of shifting, but since the central lines for both front and rear tooth faces and tooth top of the parallel moving tooth (tooth thickness is about 1.8-2 mm) are all moving in parallel toward the smaller sprocket a distance about 1 mm, when the internal chain pitch (its internal width is only about 2-2.5 mm) of the chain is engaged with the parallel moving tooth, a colliding noise probably would be generated between the chain plate of the internal chain pitch and the parallel moving tooth under a high speed running. And, in this parallel moving structure of the prior art, since the cross-sectional edge of the parallel moving tooth (tooth contour) traditionally must be formed into a R angle (round arc angle) structure of round arc shape instead of right angle edge because of the stamping manufacturing procedure, and again in a traditional chain, a R angle (round arc angle) of round arc shape is formed at the edge of the side head portion of the chain plate for the external chain plate, and additionally, for this kind of parallel moving tooth of prior art for auxiliary chaining-up, when any kind of chain shifting path is under proceeding, the side head portion of the chain plate of the external chain plate for the chain is needed to xe2x80x9chold againstxe2x80x9d the edge of the tooth contour of the parallel moving tooth for raising up the chain and completing the chaining-up motion. However, since no matter whether the side head portion of the chain plate or the tooth contour of the parallel moving tooth are all round arc shape structure, so it is impossible to be inter-held-against with each other effectively, furthermore since the tension of the chain is concentrated on the contacting points between the chain plate side head and the parallel moving tooth because of lever principle, the side head portion of the chain plate is further made to be easily glided off from the parallel tooth. Furthermore, since the position for the parallel moving tooth to hold against the chain is relatively high (i.e., the position for holding against the chain is almost at the tooth point), obviously, the higher is the holding against position, the greater is the opportunity for falling down. Therefore, in actual operation, it is often happened that the chain plate side head falls down from the tooth contour to cause the failure situation of chaining-up (or shifting), especially when the chain is under the state of high load (or high tension), the probability for shifting failure is still relatively higher, therefore a further improvement room is still existent.
Therefore, the main object of the present invention is to address a bicycle large sprocket set teeth contour structure, which has both advantages of assisting the shifting job for two kinds of chain path and lower manufacturing cost by stamping manufacturing procedure to produce projection butting surface, and which further may overcome the embarrassment situation of easily shifting failure for the parallel moving tooth structure of the prior art when a kind of chain is needed a tooth point to raise itself for shifting path, and the shortcoming of easily generating noise during high speed operation.